When an incremental position transducer is used for measuring positions, a periodic scale structure is scanned by a scanning unit provided for that purpose, to produce, on the output side, at least two periodic analog scanning signals, which are out-of-phase with each other. These signals are used for determining the relative position of the scale and the scanning unit in a downstream evaluation unit. In this manner, it is possible to record the relative position of two components which are movable relatively to one another, one of which is coupled to the scale and the other to the scanning unit, for example the relative position of two machine parts of a machine tool that are movable relatively to one another. In this case, a numerical control of a machine tool is used as an evaluation unit.
The accuracy of the positional determination using a position transducer of this kind is dependent, in particular, on the quality of the generated periodic scanning signals. In this connection, in dependence upon the physical measuring principle underlying the particular position transducer, various error sources are significant. In optical measuring systems, inaccuracies in the reflective or transmittive graduation structures of the scale can degrade the signal quality. With respect to other physical measuring principles, in particular magnetic position transducers, variations in the scanning distance or in the temperature can influence the measuring results.
Errors of this kind may have a disadvantageous effect, particularly when the scanning signals undergo a subsequent interpolation where the aim is to further electronically subdivide the analog scanning signals. It is, after all, a prerequisite for the interpolation to have an ideal form of the analog scanning signals, as well as an ideal relation between these (e.g., a precisely defined phase shift, as well as constant amplitude values). Interfering errors are considered, above all, to include different amplitude values of the two out-of-phase scanning signals, a phase shift that deviates from the predefined ideal phase shift, as well as a direct-voltage offset of the two periodic scanning signals.
To correct such deviations in the scanning signals from the ideal signal shape, it is conventional to link the scanning signals in a correction unit to correction parameters which convert the faulty scanning signals into ideal scanning signals (compare German Published Patent Application No. 197 12 622). To this end, for every signal query of the evaluation unit, correction data must be made available which are linked to the scanning signals to implement the planned correction. For this, correction data are converted into actuating signals which effect the appropriate correction of the scanning signals. Here, the signal request (or fetch operation), in which the evaluation unit requests new scanning signals (measured values of the position transducer system), triggers, in each instance, the generation of the correction data for evaluation. This takes place in dependence upon active values of the scanning signals. The signal requests can follow on the basis of defined query grids, as described, for example, in German Published Patent Application No. 197 12 622. The generation of correction data is subordinate to the evaluation unit's request for new scanning signals to be corrected and is tied to corresponding cycle times of the signal requests.
It is an object of the present invention to provide a method and a device for correcting the scanning signals of incremental position transducers that may be further improved with respect to generating correction data for the scanning signals.